1. Field of the Invention
The present invention relates generally to an improved underwater pelletizer in which a molten polymer is extruded through die orifices in a die plate in the form of continuous strands that are cut into pellets by a rotating knife holder with knives that engage the die plate. The die plate and the knife assembly are housed within a water housing so that the pelletizing takes place under water. The improved pelletizer provides a series of heat transfer tubes to transfer heat from the inlet of a transition piece where the molten polymer enters the extruder die so that heat is transferred more efficiently from the extruder forwardly to the die face and the die plate to thereby facilitate extrusion of the polymer through the die orifices. The novel use of heat transfer tubes in this setting provides for an efficient use of the heat of the molten polymer.
It is useful to have the molten polymer leaving the die orifices as warm as possible to provide uniform viscosity of molten polymer at the orifice to provide uniform strands and, in turn, uniform pellets. The warmer the polymer, the less chance there is of having die "freeze offs" when the water contacts the polymer.
The improved pelletizer of the present invention also has ceramic inserts and/or inserts of other insulating materials that surround the passages that feed the molten polymer to the die plate in which the orifices are formed. These ceramic inserts and/or inserts of other insulating materials insulate the passages near the die plate to inhibit cooling of the molten polymer as it is forced through the die plate.
The improved underwater pelletizer of the present invention also provides for water to be conducted to the cutter through a hollow shaft that supports the cutter and turns it so that the cutting blades abut the die orifice. Water from the hollow shaft is expelled from the cutter blade holder in advance of each cutter blade through water nozzles formed in the cutter blade holder. This arrangement prevents cavitation of the water in front of the cutter blades as the cutter blade holder turns to cut the extruded polymer.
All of these features are particularly useful in an underwater pelletizer designed to make very small pellets. The orifices through which the molten polymer is extruded are generally of a size of from 0.020-0.050 inches in diameter so that if cavitation occurs within the water housing of the underwater pelletizer, the very small pellets will agglomerate rather than being transported by the water out of the water housing as separate integral pellets.
2. Description of the Prior Art
Underwater pelletizers are well known and have been in use for many years. Certain advantages are gained by pelletizing molten polymer under water just as it leaves the die orifice. By cutting the polymer as it leaves the die orifice, uniform pellets are formed and the polymer is hardened within the water housing of the underwater pelletizer. The following United States patents are directed to various features of underwater pelletizers:
U.S. Pat No. 4,123,207 issued Oct. 31, 1978 PA1 U.S. Pat No. 4,251,198 issued Feb. 17, 1981 PA1 U.S. Pat No. 4,500,271 issued Feb. 19, 1985 PA1 U.S. Pat No. 4,621,996 issued Nov. 11, 1986 PA1 U.S. Pat No. 4,728,276 issued Mar. 1, 1988 PA1 U.S. Pat No. 5,059,103 issued Oct. 22, 1991 PA1 U.S. Pat No. 5,403,176 issued Apr. 4, 1995
Although many features of underwater pelletizers are shown in the foregoing patents, none of these patents disclose the novel features of the present invention.